MacOS: refactor sdk_path lookup

* Pull SDK lookup code into a new `locator` class, which caches its
  results
* SDKLocator only queries one SDK location, not all SDK locations
* Build a map of all installed SDKs inside that location, instead of
  just the requested SDK
* Ask xcrun for --show-sdk-platform-path first so that all SDKs can be
  found, instead of asking xcodebuild for a specific SDK
* Add a new `SDK` class, which tracks the version and the prefix; add a
  new `MacOS.sdk` method which returns an `SDK` instance instead of a
  bare path; MacOS.sdk_path still returns a bare path